1. Field of the Invention
This invention relates to the field of integrated circuits. More particularly, this invention relates to the control of the power supply to logic circuitry within integrated circuits.
2. Description of the Prior Art
It is known to provide integrated circuits with a virtual power supply rail for supplying power to logic circuitry. The virtual power supply rail is connected via header of footer transistors to the main power supply and by switching on and switching off these header or footer transistors the logic circuitry may be connected or disconnected from the power supply. This technique is useful in removing the power supply to logic circuitry which is inactive so as to reduce the power consumption of the integrated circuit.
When it is desired to switch from the inactive state to the active state, the header and/or footer transistors are switched on. This can result in an excessive in-rush current within the integrated circuit as the capacitance associated with the logic circuitry and the virtual power rails charges to the supply voltage. Excessive in-rush currents can produce erroneous operation and damage the integrated circuit. For this reason, it is known to take measures to reduce the in-rush current when switching on the power supply to a virtual power rail. Known techniques for reducing the in-rush current include only switching on a few of the header and/or footer transistors at first and then gradually switching on the remainder of the header and/or footer transistors in a sequence triggered by control signals tapped from a delay line. A problem with this approach is that the control of the switching on of the header and/or footer transistors must be performed with a sufficient margin that even in corner cases of process, voltage and temperature characteristics of an individual circuit the in-rush current will not exceed an acceptable level. This safe margining has the result that the switching on of the header and/or footer transistors will typically be slower than would be possible resulting in a disadvantageous increase in the latency associated with switching on the power supply to the virtual rails.